1. Field of the Invention
The present invention relates to an electroacoustic transducer for a sonar antenna. An electroacoustic transducer is used for the emission and/or the reception of acoustic pressure waves. In emission mode, an acoustic transducer transforms an electric potential difference into an acoustic pressure wave (and the reverse in reception mode).
2. Description of the Related Art
Different types of electroacoustic transducers exist. In the following of this document, particular reference will be made to the piezoacoustic transducers of the Tonpilz and Janus-Helmholtz type. Those transducers comprise a piezoelectric motor, constistuted generally of a stack of piezoelectric ceramics and electrodes, such piezoelectric motor being connected, on the one hand, to a counterweight, and on the other hand, to a horn. The piezoelectric motor, counterweight and horn assembly is connected to a prestressing rod and forms a resonator, the resonance frequency of which depends in particular on the dimensions of the horn, the motor and the counterweight.
The piezoacoustic resonator is generally placed in a sealed protective housing. The outer face of the horn is in direct contact with the immersion medium or placed behind an acoustically transparent membrane. The inner cavity of the housing is filled either with air or with a fluid chosen so as to provide a good acoustic impedance without impedance loss or discontinuity. The fluid used is generally oil. When the cavity is filled with air, the acoustic coupling between the transducer and the immersion medium is made by the outer face of the horn. When the cavity is filled with oil, the acoustic coupling between the transducer and the immersion medium is made by the horn through the oil and the housing. The immersed transducer transforms the vibration wave of the resonator into an acoustic pressure wave that propagates through the immersion medium.
An electroacoustic transducer permits to sound an acoustic echo. The specific response of a transducer depends on the frequency, on the bandwidth and on the direction of the echo with respect to the emission/reception axis of the transducer. In bathymetry applications, the transducer is placed vertically so as to sound the echo coming from the sea floor. It is then essential to sound the acoustic waves in a precise direction. Indeed, the secondary echo sources generate noise and reduce the device sensitivity.
A directivity diagram represents the acoustic intensity as a function of the direction of measurement (angularly registered). The directivity diagram indicative of the response of a Tonpilz-type transducer as a function of the direction with respect to the transducer acoustic axis is schematically shown in FIG. 2. As this diagram 12 is symmetric with respect to the acoustic axis 7 of the transducer (axis 0-180°, only half of the diagram is shown. The curve of this diagram is a curve of acoustic intensity level. It can be observed in the diagram of FIG. 2 a primary lobe 13 centred on the acoustic axis 7 of the transducer and oriented in the direction X toward the horn front. The diagram of FIG. 2 also shows a rear lobe 14, on the acoustic axis and in the direction X′ opposite to the main lobe 13. It can also be observed in FIG. 2 parasitic secondary lobes 15, 15′, 15″, in directions comprised between 40° and 140° with respect to the acoustic axis. The presence of the secondary lobes impairs the directivity of the transducer, which receives and/or emits an acoustic energy in directions different from the direction X of the transducer axis toward the horn front.
The Tonpilz-type transducers operate at frequencies between 1 kHz and 800 kHz. The problem of the secondary lobes appears when the characteristic dimension of the emitting face is of the order of or higher than the working wavelength. The wavelength λ being defined as being related to the frequency f by the relation λ=c/f, where c is the speed of the acoustic wave in the immersion medium (the speed of sound in sea water is about 1500 m/s). The problem of the secondary lobes thus appears more easily at high frequencies>50 kHz (because the wavelengths become of the order of the centimeter).
These secondary lobes are generally attributed to an imperfect decoupling between the piezoelectric motor and the housing, for which reason they are called “housing lobes”. Moreover, it is known that the pressure forces in deep immersion produce deformations and do not permit a decoupling of the motor and the housing.
Another type of transducer is derived from the Tonpilz structure; it is the Janus-Helmholtz-type transducer. Indeed, a Janus-Helmholtz transducer comprises two piezoacoustic motors aligned along a same axis and fixed to a central counterweight, each piezoacoustic motor being connected to a horn by a prestressing rod. The two horns are thus located at the opposite ends, on the axis of the device, and are symmetric with respect to a plane transverse to the axis. A Janus-Helmholtz transducer makes it possible to work at lower frequencies (from 150 Hz to 20 kHz) than a Tonpilz-type transducer.
The directivity diagram of a Janus-Helmholtz-type transducer operating at very low frequency (from 150 Hz to 20 kHz) is generally very little directive. This diagram is symmetric with respect to the transverse plane of symmetry. However, it has two power maxima on the transducer axis, in the front direction of each horn. But the power emitted or received in the direction transverse to the acoustic axis may also induce disturbances. Moreover, when a Janus-Helmholtz transducer is used at a relatively higher frequency, secondary lobes also appear.
Known solutions exist to improve the directivity of an electroacoustic transducer. The counterweight of the transducer acts as a vibration node and is thus a fixed point that is important for the transducer directivity. Therefore, the transducer directivity is improved by connecting the counterweight to the housing by a metal plate (aluminium, stainless steel, steel . . . ).
However, the secondary lobes in site around the normal to the acoustic axis are major limitations for a sonar antenna, and that whatever the type of transducer used (cf. FIG. 2). Indeed, these secondary lobes cause the presence of surface echoes and significantly deteriorate the shadow contrast of the system.
Tools for modelling the frequency response of a Janus-Helmholtz-type transducer exist, but those tools do not manage to perfectly simulate the behaviour of a transducer.